A medical device that delivers electrotherapy to a patient stores energy, and delivers the stored energy to the patient. An external defibrillator, for example, delivers energy to a patient whose heart is undergoing fibrillation and has lost its ability to contract. Ventricular fibrillation is particularly life threatening because activity within the ventricles of the heart is so uncoordinated that virtually no pumping of blood takes place. An electrical pulse delivered to a fibrillating heart may stop the fibrillation and allow it to reestablish a normal sinus rhythm.
An external defibrillator applies a defibrillation pulse via electrodes placed upon the chest of a patient. When a switch is closed, current flows between the electrodes and the defibrillator delivers at least some of the stored energy to the patient. The dosage of energy delivered is generally between 2 and 300 joules. The dosage of energy delivered may be selected by an operator of the external defibrillator, or may be selected by the defibrillator from a preprogrammed progression of dosages. In some cases, the patient may need multiple shocks, and increasing dosages may be delivered with each shock. The dosage of energy delivered when the patient is a child is generally less than when the patient is an adult.
The efficacy of a particular electrotherapy dosage for a particular patient is influenced by physiological characteristics of the patient, such as the transthoracic impedance of the patient. Conventional defibrillators are often specified for and tested with a standard load, such as 50 ohms, that represents the transthoracic impedance of a standard patient. Further, operators of defibrillators often select dosages, and progressions are often preprogrammed based on formulas, tables, or protocols that assume a standard patient with a standard transthoracic impedance. The actual transthoracic impedance of patients, however, can vary greatly in a range from 20 to 300 ohms, though most patients typically fall in a range from 25 to 180 ohms. Further, different classes of patients exhibit different ranges of transthoracic impedances. For example, an average transthoracic impedance for adults is approximately 80 ohms, while an average transthoracic impedance for children is about 60 ohms.
The efficacy of a defibrillation pulse is a function of the current that flows between the electrodes and the width of the defibrillation pulse. The current that flows between the electrodes is a function of the voltage between the electrodes and the transthoracic impedance of the patient. For a given voltage level, a patient having a lower transthoracic impedance than the average will experience a greater current flow than a patient with the average impedance, and a patient having a higher transthoracic impedance than the standard will experience a lesser current flow than a patient with the average impedance. Too little current flow resulting from a defibrillation pulse may not be effective in defibrillating the heart of the patient, while too great a current flow can be inefficient, and more importantly, may damage the tissue of the patient.